In the field of electronic device materials, heat-curable resins typically having an epoxy resin as a main component are widely used to improve electrical reliability, such as heat resistance, insulating properties, adhesion and the like. Among such resins, from the perspective of heat resistance, electrical properties and mechanical properties, resins compliant with FR4 grade (NEMA standards) which have a brominated epoxy resin as a base and are designed to have a glass transition temperature (Tg) of 130 to 140° C. are widely used.
However, in recent years, the demands for making electric appliances smaller, lighter and more functional have been increasing, resulting in demands for more highly integrated and more reliable semiconductors and laminates. There is a particular need for a material which can withstand being heated in a connection process using a lead-free solder having a high softening point and the heat generated by the increased integration.
On the other hand, while it has been common to confer an electric appliance, especially a printed wiring board, with flame retardance by using a halogenated compound, it has recently been pointed out that there is a possibility of a tiny amount of dioxins forming if the printed wiring board is strongly heated. While it is yet to be confirmed whether this is true, to prevent in advance environmental problems in case of fire or during heating when recycling, there is a need to ensure flame retardance without the use of a halogenated compound.
Examples of substitute technologies for halogenated compounds include mixing and dissolving a phosphorous-containing compound such as a phosphate or the like into the epoxy resin component, or mixing and dissolving in a metal hydroxide such as aluminum hydroxide or the like. However, in both of these cases a large amount has to be blended in order to ensure flame retardance, which can cause the heat resistance, water resistance and adhesion of the cured resin to deteriorate.
According to research carried out by the present inventors regarding flame retardants which are well known for non-halogen flame retardance, it was discovered that for a high performance printed wiring board which handles high-speed electrical signals, if the added amount of the non-halogen flame retardant is changed, the printed wiring board can no longer function unless the wiring design is changed, because the electrical properties of the entire resin are typically altered by the addition. The amount of a flame must be changed, for example, when the number of wiring layers is increased to improve the functions of a multilayer printed wiring board, which is an important example of a high-performance printed wiring board, thereby increasing the thickness of the printed wiring board. Thus, consideration has to be given to practically important cases.
For example, Patent Document 1 discloses a resin composition comprising an epoxy resin which has an oxazolidone ring and a phosphorous compound such as tris(2,4-di-t-butylphenyl)phosphite. Patent Document 2 discloses a resin composition comprising a phosphazene compound and an epoxy resin such as a phenol novolac epoxy resin or a cresol novolac epoxy resin. However, while these resin compositions satisfy the required standards for heat resistance, they fail to exhibit sufficient flame retardance when the thickness of the laminate increases.    Patent Document 1: JP-A-8-127635    Patent Document 2: JP-A-2001-335676